The bank's ATM indicated that I had a positive balance. But when I wrote a check, the bank charged an overdraft fee. How is this possible?

Many transactions are processed overnight. These transactions may not be reflected in an available balance, such as an ATM balance, that you see during the day.

This is why it's important to keep a current and accurate check/transaction register and balance it to your monthly statement. A bank's online, telephone, or ATM balances are for information purposes only—they do not replace your check/transaction register.

On checking accounts, national banks generally post deposits before withdrawals. However, banks aren't required by law to do this. In addition, banks may establish a cutoff time for deposits made at a branch or through an ATM. Deposits made after the cutoff time may be treated as having been made on the following business day.

For example, a deposit made after the Friday afternoon cutoff time would be treated as if it were made on the following Monday. So any items with next-day availability would be available on Tuesday.